(a) Field of the Invention
This invention relates to exhaust gas catalytic conversion systems.
(b) Description of the Related Art
Many exhaust gas catalytic conversion systems are composed of a manifold catalyst directly coupled to an exhaust manifold of an engine and an underfloor catalyst disposed downstream of the manifold catalyst in the direction of exhaust gas flow. The manifold catalyst mainly converts exhaust gas during a cold time of the engine and is made of a catalyst material having a high low-temperature activity. On the other hand, the underfloor catalyst mainly converts exhaust gas during a warm time of the engine during which the engine operating conditions variously change to fluctuate the A/F ratio. Therefore, the underfloor catalyst is expected to have the capacity to efficiently convert exhaust gas even if the period or amplitude of A/F ratio fluctuations largely changes. To attain this, ceria and/or one or more other oxygen storage components are conventionally used as promoters to absorb fluctuations in A/F ratio and extend the A/F ratio window for a three-way catalyst (A/F ratio width within which the catalyst effectively works). The assignee has proposed various catalysts.
For example, Published Japanese Patent Application No. 2005-161143 describes the use of, as it were, particles of Rh-supported and Rh-doped mixed oxide (composite oxide) obtained by preparing Rh-doped mixed oxide particles in which Rh (rhodium) is doped at or between crystal lattice points of a cerium-zirconium-neodymium (CeZrNd) mixed oxide and supporting Rh also on the surfaces of the Rh-doped mixed oxide particles acting as an oxygen storage component.
Furthermore, Published Japanese Patent Application No. 2006-035043 describes the use of a catalyst in which Rh-doped mixed oxide particles and Pt-supported active alumina particles are mixed or a catalyst in which the two different particles are disposed in two different catalyst layers and these catalyst layers are laid one on the other.
Furthermore, Published Japanese Patent Application No. 2006-043488 describes a lean NOx catalyst obtained by preparing Rh-doped mixed oxide particles in which Rh is doped at or between crystal lattice points of a CeZr mixed oxide and supporting Pt on the surfaces of the Rh-doped mixed oxide particles acting as an oxygen storage component.